Xo. 7. J WURTZ — THEORY OF ATOMS. 3£1 



out, adds Dumas, modifying the fundamental properties. This 

 proposition encountered at first the most violent contradiction. 

 How could chlorine take the place of hydrogen and play its part 

 in combinations? These two elements, said Berzelius, are en- 

 dowed with opposite properties, and if the one is lacking the 

 other cannot supply its place ; for, in short, they are two inimical 

 brothers, little disposed and by no means fit to be kept in the same 

 house. These critics and many others have not prevailed against 

 facts. The theory of substitutions has come triumphantly out 

 of this great discussion, which marks a date in the history of our 

 science. Its natural development has gradually introduced into 

 it new ideas on the constitution of chemical compounds, on the 

 mode of combination of the elements which they contain. 



These ideas have come to light by various ingenious compari- 

 sons. Laurent considered organic compounds as formed of nuclei 

 with appendages, both the one and the ether admitting into their 

 structures atoms grouped with a certain symmetry. Dumas com- 

 pared them to edifices of which the atoms constitute, in a manner, 

 the materials. Hence the graphic but frequently correct ex- 

 pression, of molecular edifices capable of being modified, in 

 certain cases, by the substitution of one part for another, and 

 which, in other cases, the shock of powerful reagents may shatter 

 to pieces. In both conceptions the chemical molecules were 

 regarded as forming a whole. A little later Dumas compared 

 them to planetary systems; and here he veritably shot ahead 

 of his time in giving us a glimpse of groups of atoms maintained 

 in equilibrium by affinity, but carried along by movements, as 

 the planets of a solar system are acted upon by gravitation and 

 carried into space. It is in these movements of atoms and mole- 

 cules that at a later period the source of the physical and chemi- 

 cal forces must be sought for; but I must not anticipate. I 

 have attempted to show how the ideas on chemical combinations 

 have been gradually modified under the double influence of the 

 atomic hypotheses and of facts brought to light by the French 

 school concerning their reciprocal replacement in combinations. 

 Forming a whole, more or less complex, the molecules of organic 

 substances may be modified by substitution and give rise to a 

 multitude of derivatives which naturally attach themselves to the 

 mother substance. The latter serves them as a model or type. 

 The typical idea thus introduced into science very soon occupied 

 A large place. It first brought to it important elements of classi- 



